President Obama / Mandel Ngan, AFP/Getty Images

by Peter Eisler, Kevin Johnson and Aamer Madhani, USA TODAY

by Peter Eisler, Kevin Johnson and Aamer Madhani, USA TODAY

WASHINGTON - The FBI said Wednesday it has arrested a Mississippi man "believed to be responsible'' for mailing letters that tested positive for poisonous ricin to President Obama, a senator and another official.

Paul Kevin Curtis was arrested at his home in Corinth, Miss., the FBI said.

Curtis is "the individual believed to be responsible for the mailings of the three letters sent through the U.S. Postal Service which contained a granular substance that preliminarily tested positive for ricin,'' the FBI said in a statement.

The White House and Capitol Hill were on alert after initial tests Wednesday showed that suspicious letters contained ricin, a deadly toxin.

As federal authorities investigated the letters to Obama and Sen. Roger Wicker, R-Miss., several other senators reported that they, too, had received suspicious mail. The reports prompted U.S. Capitol Police to close portions of two congressional office buildings for a brief time.

Preliminary tests on the granular substance in the letter to Obama showed evidence of ricin, but additional lab analysis is underway to confirm that finding, according to an FBI statement. Similar testing is being done on the letter to Wicker, and results can take 24-48 hours.

Both letters bore a Memphis postmark, law enforcement authorities said. In an intelligence bulletin obtained by the Associated Press, the FBI reports that both letters say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both are signed, "I am KC and I approve this message," wording that candidates use at the end of their broadcast political ads.

Ricin can be deadly if ingested or inhaled, but it's generally considered ineffective as a weapon for mass terrorism because it is very difficult to put it into an airborne form.

The letters to Obama and Wicker both were intercepted at off-site screening facilities where official mail is checked for potential contaminants. Obama was briefed on the suspicious letters Tuesday night and again Wednesday morning, said White House press secretary Jay Carney.

Both the FBI and the White House stressed that there is no indication of any connection between the letters and Monday's twin bombings at the Boston Marathon. Carney cautioned that Americans shouldn't jump to any conclusions.

"Before we speculate or make connections that we don't know â?¦ we need to get the facts," Carney said.

The letter to Wicker was discovered first, on Tuesday, and Capitol Police said it contained a "white granular substance." The letter to Obama was revealed Wednesday morning.

Another suspicious package was received Wednesday morning at the Washington offices of Sen. Richard Shelby, R-Ala., according to Shelby's spokesman Jonathan Graffeo. He said the package is being investigated by Capitol Police and it was not known if it was similar to the ones addressed to Obama and Wicker.

Sen. Carl Levin, D-Mich., issued a statement saying that a suspicious letter was received Wednesday morning at his Saginaw, Mich., field office. He said the staffer who received the letter did not open it and turned it over to authorities, who are investigating and testing the letter for toxins.

"We do not expect to learn at least a preliminary result of those tests until late tonight or tomorrow," Levin said in a statement. "The staffer who discovered the letter is being kept overnight at a local hospital for precautionary reasons, but has no symptoms. We do not know yet if the letter has any connection to suspicious mail sent to other public officials."

Also Wednesday, authorities evacuated the Phoenix office of Sen. Jeff Flake, R-Ariz., after receiving reports of suspicious letters. Flake later announced that investigators had determined that there was no threat.

All routine mail to Congress and the White House is screened off-site, FBI spokesman Paul Bresson said, but those tests can produce inconsistent results. If the tests indicate the possibility of a biological agent, the material is sent to an accredited laboratory for further analysis, and only those tests can confirm the presence of a biological agent.

As news of the letters spread Wednesday, tensions ran high at the Capitol. U.S. Capitol Police alerted congressional staff just after noon that they were investigating a suspicious package in the atrium of the Hart Senate Office Building, as well as suspicious envelopes received in upstairs offices of both the Hart building and the nearby Russell Senate Office Building. The message advised staff that they were not required to stay in their offices, but to avoid the areas where the suspicious packages were being investigated.

An announcement in the buildings just before 1 p.m. said that test results on the packages were negative and closed off areas had reopened. Scott Ongill, a Senate staffer, said such alerts are not unusual. "There is a couple, three (alerts) a week around the Capitol campus. It's not that uncommon an occurrence. They're doing due diligence." He said they usually end up being tourists who left something.

Ricin is derived from the same plant used to produce castor oil. Because it is not readily put in the form of an aerosol that spreads easily through the air, it is considered far less worrisome as a weapon than anthrax, the substance mailed to members of Congress in shortly after the Sept. 11, 2001, terror attacks in an episode that killed five and sickened 17 others.

"It's high school chemistry to make it, but it's not a serious bio threat," said Randall Larsen, former executive director of the WMD Commission, which was set up by Congress to investigate threats posed by weapons of mass destruction. "You won't find bio-terror experts getting all upset by ricin. It's usually (used by) nut jobs doing something to try to get attention from people."

Michael Osterholm, a former bioterrorism adviser to the George W. Bush administration who heads the Center for Infectious Disease Research and Policy at the University of Minnesota, called the interception of the ricin letters "a success story."

"It's a horrible situation that it happened but it's a success that it was found," Osterholm said. "The ability to identify ricin in a letter was not in place before 2001. The U.S. mail is being tested for things like this. This was not a letter that got opened and someone said, 'Oh my gosh.'"

Handling a letter would be unlikely to give someone a lethal dose of ricin, Osterholm says. The poison is most dangerous when inhaled; getting it on one's fingers wouldn't pose that sort of risk. However, if someone is poisoned with ricin, there is no treatment and it can kill in one to three days.